This invention pertains to non-aqueous thermosetting epoxy-amine protective coatings and particularly to the inclusion of additive amounts of organic acids to provide thixotropic or pseudoplastic characteristics to the coating.
The rheological or flow properties of liquid organic protective coatings are generally designed to provide a controlled degree of non-dripping, non-sagging or non-running behavior such that after application to a vertical surface by spraying or brushing, a uniform thickness of coating is maintained on the vertical surfaces without dripping, etc. while the coating dries or cures to a hard film At the same time, however, it is also necessary that the liquid coating have sufficiently low viscosity so that it may be easily atomized by spraying or spread over a surface by brushing or rolling. Such rheological behavior where a viscous coating displays a lower viscosity when sheared by spraying or brushing in known as pseudoplasticity or more commonly as thixotropy. Pseudoplastic/thixotropic coatings possess the desirable characteristic of sufficiently high viscosity and resistance to dripping or sagging after being applied to a vertical surface but will exhibit a lower viscosity when sheared during the course of spraying or brushing such that atomization and spreading may be accomplished without undue effort.
Liquid organic coatings, including epoxy-amine coatings ordinarily are rendered thixotropic/pseudoplastic through the inclusion of certain particulate materials such as fumed silica, treated bentonite clays, or hydrogenated castor waxes. Pigmented epoxy-amine coatings where the epoxy resin and amine curing agent are mixed immediately before use are often formulated to contain such particular materials to provide resistance to sagging or dripping from vertical surfaces while maintaining good atomizing and brushing properties. However, such particulate materials are costly and require considerable care and energy to disperse into an organic coating. Illustrative comparative examples of these prior suggestions appear in the examples hereinafter.
Thermosetting epoxy-polyamide coatings are disclosed in U.S. Pat. No. 4,771,088 where the coating contains above about 1% by weight of a cationic stabilizing additive comprising an ethoxylated quaternary ammonium chloride salt based on the weight of the epoxy-polyamide polymeric solids. This patent teaches that viscosity and sag resistance are maintained during the catalyzed pot life of the coating when the two-component epoxy resin and polyamide are mixed together.
It now has been found that a thermosetting epoxy-amine protective paint coating containing minor amounts of an organic acid renders the coating thixotropic or psuedoplastic to enable brushing, rolling or spraying of the coating with minimum applied shear force. Expensive fumed silica or Bentone clays are avoided. In accordance with this invention, the inclusion of at least 0.05% by weight of an organic acid to the epoxy-amine coating provides the necessary thixotropic properties. Pigmented epoxy-amine coatings can be formulated to be pseudoplastic/thixotropic and relatively easy to manufacture at lower cost through the inclusion of the minor amount of organic acid and preferably fatty acid or polymerized fatty acid. Such modified coatings exhibit good spraying and brushing behavior while also resisting sagging and dripping when applied to vertical surfaces. Sag resistance of paints can be measured by ASTM D4408-84. Thixotropic properties can be measured by ASTM D4400-84. Thixotropic this invention will become more apparent from the detailed description of the invention and the illustrative examples.